The present invention relates to a superconductive magnet coil arrangement for generating a homogeneous magnetic field in a volume under examination, comprising a magnet coil having two first windings of a superconductive wire which are disposed adjacent the ends of the magnet coil and which are supported on a hollow supporting body enclosing the volume under examination, and comprising further at least one additional winding of a superconductive wire which is arranged in the area between the first windings and which is supported by the said supporting body, the windings being fixed against displacement in the axial direction.
A magnet coil of this kind has been known for example from EP-A-0 293 723 and EP-A-0 285 861. The supporting body, which displays a substantially cylindrical shape, comprises a plurality of winding chambers each containing one winding made from a superconductive wire. Flanges left between the winding chambers fix the windings rigidly in the axial direction. From EP-A-0 118 807 (GE) it has been known to arrange the windings radially inside a supporting body made from aluminium, and to provide low annular stops in order to thereby prevent the windings, i.e. the entire winding packages, from being displaced due to magnetic forces.
In the case of superconductive coils of the kind described above, Lorentz forces, which act on the superconductive wire in the axial direction of the coil, are encountered due to the existing radial components of the magnetic field. These forces sum up in each winding, which comprises a plurality of turns. At the axial inner end of the winding chamber, the forces are then transmitted to the flange and, via the supporting body, to the next winding chamber in the form of a pressure. Such a transmission of pressures to other winding chambers is not encountered in a disturbing manner only in such cases where a maximum of two windings are provided and arranged at an axial distance one from the other. The axial pressure sums up towards the middle of the coil and leads to a reduction in length of the supporting body in the area between the two axially outer windings, as a result of the axial pressures encountered. The highest forces, however, arise at the coil ends because there the radial components of the magnetic field reach their maximum if the current has the same intensity throughout.
It has been found by experiments that the pressure in the winding core must not exceed a certain maximum value which is predetermined by the properties of the material of the wire and by the winding technique. If the pressure gets excessively high, a coil of this type can no longer be operated at very high field strengths because the process of building up a high magnetic field Will be terminated, during charging of the coil, at some point or other by instable displacements of the winding turns. For a series of homogeneous coils with relatively high field strengths it is the pressure which limits the magnetic field strength. The described transmission of the axial pressure over the full length of the winding is also encountered in the case of the before-mentioned EP-A-0 118 807, because of an intimate interference fit existing between the windings, which are not delimited by lateral walls of the winding chambers, and the supporting body.